A three-phase brushless dc motor can typically be thought of as having a stator with three coils, U-phase, V-phase, and W-phase, and a rotor with permanent magnets. As shown in FIG. 1, the permanent magnets comprise a main magnet 10 which has repeatedly alternating North and South magnetic poles for providing the necessary torque for motor rotation and a sub-magnet 12 which has repeatedly alternating North and South magnetic poles, for controlling motor speed. The ratio of the magnetic poles with respect to the main magnet and the sub-magnet preferably is 1 to 3.
The three-phase brushless dc motor enables motor rotation by sending currents through the coils which create a magnetic field which in turn generates a torque for motor rotation. In order to maintain the rotation in one direction, magnetic fields of the rotor are detected with Hall sensors, resolvers, or photo-encoders and then the direction of electric current flowing in each stator coil is changed based on the intensity of the detected magnetic fields. This changing of direction of electric current flow is called commutation.
Also in FIG. 1, Hall signals detected from the magnets by Hall sensors during motor rotation are shown. The Hall signals are periodic sine waves expressing the magnetic field intensity of the main magnet 10 and sub-magnet 12. The Hall signal of the main magnet 10 is expressed as HM, and the Hall signal of the sub-magnet 12 is expressed as HS.
A Hall sensor outputs original Hall signals ("positive Hall signals") and signals with 180 degree phase difference ("negative Hall signals") to the positive Hall signals. It should be noted that only positive Hall signals are shown in FIG. 1. The Hall signals HM of the main magnet 10 ("main Hall signals") and the Hall signal HS of the sub-magnet 12 ("sub-magnet Hall signals") have a period ratio of one to three since the ratio of magnetic polarities of the main magnet 10 and the sub-magnet 20 is assumed to be one to three.
Generally, a three-phase brushless dc motor with three Hall sensors has been used to obtain three Hall signals in order to produce proper commutation of currents in the stator coils. A modification to the above brushless dc motor, i.e., a three-phase brushless dc motor with one Hall sensor, was introduced. However, under this technique, the commutation is done based on the phase of Hall signals irrespective of its magnitude (voltage value). As a result, sparks may occur during the switching process for commutation which can increase electromagnetic interference. Snubbers are used to prevent sparks.
Therefore, a preferred three-phase brushless dc motor still is a motor with three Hall sensors to generate three main Hall signals, each signal having 120 degree and 240 degree phase differences with respect to other two signals. This is done in order to take into account of both magnitude and phase of the Hall signals in performing commutation.
A driving circuit for motors using three Hall signals is shown in FIG. 2. The driving circuit comprises three emitter coupled pairs 20, 22 and 24; a stator 28 having U-phase coil, V-phase coil and W-phase coil; and an inverter 26. The inverter 26 controls current direction in each coil of the stator 28 by sequentially turning on and off its switches which is connected to the stator 28, based on currents Ic1-Ic6 flowing through the output terminals of the emitter coupled pairs 20, 22 and 24.
The three emitter coupled pairs 20, 22 and 24 all have identical structure, so the structure of emitter coupled pair 20 will now be explained as an example. The emitter coupled pair 20 has two npn type transistors P1 and P2. The emitters of the transistors P1 and P2 are connected to a common node; the base of the transistor P1 and the base of the transistor P2 receive a positive Hall signal HM1+ and a negative Hall signal HM1-, respectively; and the collector of the transistor P1 and the collector of the transistor P2 receive the current Ic1 and the current Ic2, respectively. A current source IEE1 is inserted between ground and the emitter.
The voltage difference of the positive main Hall signal HM1+ and the negative main Hall signal HM1- being applied to the bases of the emitter coupled pair 20 determines amount of the currents Ic1 and Ic2 flowing through the collectors of the transistors P1 and P2. The current flow amount in other two emitter coupled pairs 22 and 24 are determined similarly by the voltage difference of the positive main Hall signal HM+ and the negative main Hall signal HM-.
A first, a second, and a third Hall voltage difference signals VHU, VHV and VHW of the positive and the negative Hall signals are shown in FIG. 3. For example, the first Hall voltage difference signal VHU is voltage difference of the first positive main Hall signal HM1+ and the first negative main Hall signal HM1-. The first, the second and the third Hall voltages differences VHU, VHV and VHW, each have a phase difference of 120 degrees and 240 degrees with the other two signals.
The currents Ic1-Ic6 of emitter coupled pairs 20, 22, and 24 increase or decrease linearly within section A. As shown in FIG. 3, section A refers to sections where the first, the second and the third Hall voltage differences VHU, VHV and VHW are within positive 50 mV to negative 50 mV with respect to the Hall bias voltage, i.e., within 100 mV with respect to the Hall bias voltage, which is about four times the thermal voltage. In other words, section A refers to sections where the voltage differences of positive main Hall signals to its negative main Hall signals are within positive 50 mV to negative 50 mV with respect to the Hall bias voltage.
In areas outside of the section A, i.e., in section B, the currents Ic1-Ic6 do not increase or decrease linearly due to the current sources IEE1-IEE3. Therefore, internal switches of the inverter 26 are turned on or off. Current directions of the stator coils are changed in the section A so commutation occurs therein. The section B corresponds to the period when commutation has been completed.
In this way, commutation is done only in the section A so soft switching is made possible by using the currents Ic1-Ic6 which are varied based on both phase and magnitude of Hall signals. Soft switching has advantages of reducing sparks during commutation. However, for soft switching, the conventional technique uses three Hall sensors which can result in increases in size and cost of driving circuit.
In video cassette recorders, floppy disk drives, and other devices which read or write data by using a motor driven at a constant speed, it is often desired to determine the starting point of motor rotation in order to read or write data without errors. Conventionally, additional mechanical elements are determine the starting point of motor rotation, i.e., an index marker. The stator additionally includes a covering cap, a protrusion on the cap, and a sensor for detecting the protrusion on each revolution of the stator and sending out an index marker per revolution of the stator. A technique using this method is described in Korean Patent Application No. 96-55802.